सीतानाथ समारम्भां रामानन्दार्य मध्यमाम्।
अस्मदाचार्य पर्यन्तां वन्दे श्रीगुरू परम्पराम् ।।
First understand their are two Shiv Samhitā, one is dedicated to Yōgic Kriyās and all and second one is dedicated to Rasopāsna of Śrī SītāRām.
The Shiv Samhitā which is available on Internet is actually Shiv Yog Samhitā but the second one is highly secretive, can only be found to the Rasik devotees of Lord Rāma (especially to Rāmānandīs).
In fact no one should have any doubt on its authenticity as it has been